The guinea pig in vitro carcinogenesis model, in which preneoplastic sequential stages can precede neoplastic transformation, the acquisition of tumorigenicity, by two years is useful in establishing the relationship of various cell surface properties, both individually and as combined factors, to progressive tumor growth. In this model, stages and factors associated with carcinogenesis are studied separate from those properties found only in tumorigenic cells. Using this model, we have found that all tumorigenic cells produce plasminogen activating proteases in amounts which correlate with the (1) tumorigenic threshold inoculum, (2) ability to induce a delayed type skin reaction in non-immune guinea pigs and (3) susceptibility to guinea pig lymphotoxin. All tumorigenic cells produced reduced amounts of fibronectin and Forssman glycolipid but the amounts produced do not correlate with the tumorigenic ability of the cells. An inhibitor of plasmin activity was produced by the same cells producing the plasminogen activating enzymes suggesting that control of local in vivo proteolysis may include an antagonistic relationship between plasminogen-activating and plasmin-inhibiting activities.